At exactly 4:50 a.m. on Sunday, September 3, 1967, every car in Sweden rolled to a stop. For ten minutes, drivers waited in eerie silence while loudspeakers counted down. Then at 5:00 a.m. the radio crackled, Now is the time to change over, and every vehicle slowly, carefully, drove across to the right side of the road. Sweden had just done the biggest traffic switch in history, all at once, on a day they called Dagen H, short for Hogertrafikomlaggningen, or the Right Hand Traffic Diversion.
For 233 years Swedes had driven on the left, but their cars were built with the steering wheel on the left side, which made overtaking tricky. Worse, every neighbor country drove on the right, so border crossings were a mess. Planning Dagen H took four years. Workers painted over millions of lines, rewired 360,000 street signs in a single night, and even repainted buses with the doors on the new side. Kids learned the new rules in school using songs, and the government printed posters of a cheerful mascot named Herr Hogerman.
Amazingly, the switch went almost perfectly. On the Monday after Dagen H, Sweden actually had fewer accidents than a normal Monday, because everyone drove super slowly and paid extra attention. Crashes went back to normal within weeks, but the careful planning became a famous example of how a whole country can change a deep habit overnight if it prepares well enough. Engineers still study Dagen H today when they design huge changes like a new currency or a city-wide subway opening.
